This invention relates to control of gas flows, and more particularly to a bleed system for releasing pressurized gas from a chamber at a selectively adjustable flow rate. The bleed system is described primarily herein for application in a runaway control for an air motor which shuts off the motor when its speed exceeds a predetermined threshold speed. However, it is understood that the bleed system may be applied in any device where pressurized gas is released at a controlled rate.
It is difficult to accurately control a release of gas from a higher pressure region to a lower pressure region, particularly for an adjustable, low-level release. Due to manufacturing tolerances, conventional bleed valves frequently exhibit inconsistent or varying sizes of flow passages. Adjustments which are intended to produce minor incremental changes in flow rates can result in large step changes. Consequently, the valves are over-sensitive, cannot be accurately calibrated, and are not repeatable in setting a selected rate of flow.
A runaway control of the prior art is disclosed in U.S. Pat. No. 5,349,895, entitled “Air Motor Control,” which is hereby incorporated by reference. That patent discloses an air motor with an expansible chamber having a reciprocable piston driving a pump for pumping materials such as lubricants, sealants, or inks. A problem of pump runaway is at times encountered, due for example to breakage of a discharge line or running out of the material being pumped. The load on the motor is reduced such that the motor speeds up and drives the pump at very high speeds. That can damage the pump and cause expensive and time-consuming spills of material. A runaway control is provided for cutting off operation of the air motor under these circumstances. The control can be adjusted so that it activates to cut off the air motor at a selectable and predetermined threshold speed (e.g., between 5 and 50 cycles per minute) which depends generally on the viscosity of the material being pumped. Adjustment of cut-off speed is effected by a bleed valve for adjusting the rate of flow from a chamber having a pressure which varies in proportion to the speed of the motor. The bleed may be adjusted to vent a maximum quantity of air from the chamber when operating the motor at an increased speed, or adjusted to vent a minimal quantity of air when operating the motor at a nominal speed.
A drawback to runaway controls is that bleed valves are prone to be over-sensitive to adjustments, as described above. It is difficult to accurately calibrate the bleed valve to obtain a desired cut-off speed or to repeat previously obtained settings.